The conventional process for producing gaseous olefins from petroleum hydrocarbons is steam cracking process, wherein the feedstocks are light hydrocarbons comprising natural gas, naphtha and light gas oil and so on. The supply of light hydrocarbons was limited as crude oil becomes heavier. Thus, more attentions are paid to processes for producing gaseous olefins from heavy hydrocarbons. Such processes, for example, include thermal cracking process of heavy hydrocarbons by using inert solid materials such as quartz or coke as heat carriers, and thermo-catalytic cracking process of heavy hydrocarbons by using alkali metal oxides or alkaline earth metal oxides as catalyst. The reaction temperatures in such processes are both greater than 800° C.                Recently the processes for producing gaseous olefins from petroleum hydrocarbons have been described in various publications, wherein using solid acidic catalysts in certain reactor style and under certain operation conditions. For example, JP 60-222,428 discloses a process using ZSM-5 as a catalyst and C5 to C25 paraffinic hydrocarbons as feedstock The process is carried out at a reaction temperature of between 600 to 750° C., with 30 wt % yield for C2 to C4 olefins. U.S. Pat. No. 3,758,403 disclosed that a catalyst comprising both ZSM-5 zeolite and large pore zeolite (e.g. X type or Y type) as active components in a ratio of 1:10-3:1 raised octane number of gasoline while increasing yield of total propylene and butylenes to about 10 wt %.        It has been reported in U.S. Pat. No. 5,318,696 that the catalyst comprising large pore zeolite and medium pore zeolite in ratio of SiO2/Al2O3 being less than 30 can be used to produce high octane gasoline, and to increase the yield of gaseous olefins, especially propylene. Said medium pore zeolite is in the form of MFI structure.        U.S. Pat. No. 4,980,053 disclosed the catalyst comprising mixture of ZSM-5 zeolite and Y type zeolite as active components can be used to increase octane number of gasoline in products and to increase yield of light olefins in reaction temperature of 500˜650° C. And light olefins are mainly consisting of propylene and butylene.        It has been reported in prior art that the process for modifying zeolite having a structure of pentasil to increase the selectivity to the reaction product. For example, phosphorus and/or metal ion can be incorporated into zeolite having a structure of pentasil (such as ZSM-5) to adjust the adsorption and catalysis properties of the zeolite.        It has been reported in U.S. Pat. No. 4,365,104 that the process for modifying ZSM-5 zeolite by using phosphorus and magnesium, for the purpose of using modified zeolite to the isomerization of xylene to increase selectivity to xylene. The incorporation of phosphorus and magnesium is to increase the shape selectivity of zeolite. However, both acidity of molecular sieve and hydrocarbons conversion reactivity of zeolite after modification are decreased.        
It has been reported in U.S. Pat. No. 5,236,880 that the incorporation of group VIIIB metals, preferably nickel into zeolite in the ratio of SiO2 to Al2O3 being greater than 5. Said zeolite is in the form of MFI or MEL structure. The catalyst disclosed in U.S. Pat. No. 5,236,880 can be used to increase the conversion of paraffinic hydrocarbons, to increase aromatics content in gasoline fraction, to increase octane number of gasoline in products and yield of gasoline.                A catalyst comprising both phosphorus and rare earth-containing silica rich zeolite having a structure of pentasil as active component has been disclosed in U.S. Pat. No. 5,380,690 and China patent publication 1,093,101A respectively. The catalyst has stronger hydrothermal stability. The conversion obtained by using said catalyst is higher than that obtained by using the catalyst comprising HZSM-5 zeolite as active component by 4-7 wt % in the reaction temperature of 580° C. Similarly, the yield of C2-C4 olefin is higher by 4˜5 wt %.        
A combination process of catalytic cracking with dehydrogenation has been disclosed in U.S. Pat. No. 5,414,181. The spent catalyst is regenerated after the catalytic cracking reaction is completed. The regenerated catalyst is deposited with carbon derived from coke precursor and then C2-C10 alkane is dehydrogenated to produce olefins.
It has been disclosed in U.S. Pat. No. 6,106,697 that a process for selectively producing C2-C4 olefin by using vacuum gas oil or residual oil as feedstock and using two-stage reactor to carry out catalytic cracking reaction. Vacuum gas oil or residual oil feedstock is fed into the first stage reactor. The feedstock is contacted with large pore zeolite catalyst under conventional catalytic cracking conditions to carry out catalytic cracking reaction to produce various products with different boiling ranges, including gasoline fractions. The gasoline fractions produced in the first reactor are fed into the second reactor and contacted with medium pore zeolite catalyst in the reaction temperature of 500-650° C., the catalyst/oil ratio of 4-10:1, and the oil vapor partial pressure of 70-280 kPa. As a result, C2-C4 olefins are produced.
It has been reported in China patent publication No. 1,255,474A a catalytic cracking process for producing light olefins from lower alkane. The process comprises contacting C4-C6 alkane with a silica rich zeolite catalyst being modified by one of the elements selected from the group consisting of alkali metal, alkaline earth metal and group VIII transition metal and having a structure of pentasil, wherein the catalyst comprises rare earth and phosphorus. The reaction is carried out in temperature of 450-580° C. and weight hourly space velocity of 5-300 hour−1. The alkali metal is preferably potassium, alkaline earth metal is preferably barium and group VIII transition metal is preferably iron or nickel.
It has been disclosed in U.S. patent publication No. 2003/0006168A1 that a catalytic cracking process for producing light olefins with a high yield from heavy oil. The process comprises the steps of contacting the heavy oil with a catalyst mixture, consisting of 60 to 95% by weight of a base cracking catalyst containing an USY type molecular sieve and less than 0.5% by weight of rare earth metal oxide, and 5 to 40%. by weight of an additive containing a shape-selective zeolite; the oil and the catalyst are contacted in a downer reactor, and are contacted under conditions so that the reaction zone outlet temperature is in the range of 580-630° C., the catalyst/oil ratio is in the range of 15-40 and the contact time is in the range of 0.1-1.0 seconds. Said process can be used to improve yield of light olefins, for example, the yield of propylene can be about 20% by weight.
It has been disclosed in EP 0,922,744A1 that a process for producing LPG and light olefins with a high yield under the reaction conditions of weight hourly space velocity in the range of 40-120h−1, catalyst/oil ratio in the range of 15-25, riser outlet temperature in the range of 530-600° C., reaction pressure in the range of 1.0-4.0 Kg/cm2 g and steam for dilution and quenching of hydrocarbon in the range of 3-50% by weight of the feedstock. The catalyst used in said process comprises 1-6% by weight of USY zeolite, 8-25% by weight of shape-selective zeolite, 0-8% by weight of bottoms selective cracking active component, 0-1% by weight of rare earth component and 60-91% by weight of non-acidic component & binder. The yield of LPG obtained by the process can be about 40-65% by weight. The selectivity of propylene and butylenes in LPG are about 40% and 45% by weight respectively.
The carbon content in the regenerated catalyst is typically required to be less than 0.1% by weight and preferably be less than 0.05% by weight during the catalytic crack process for the purpose of restoring cracking activity of catalyst.
As discussed above, the process for producing light olefins with a high yield by using phosphorus-containing silica rich zeolite having a structure of pentasil as active component has not been described in the prior art, wherein said silica rich zeolite has also been modified with group VIII transition metals, although the processes such as in which silica rich zeolite having a structure of pentasil being used as one of the active components in the catalyst have been disclosed.